(b) Description of Prior Art
Organ transplantation is presently the only alternative to alleviate organ failures and to restore or improve the function and performance of organs. However, some of the drawbacks of organ-transplant therapies, are the potential for donor-to-recipients disease transmission, the shortage and the limited availability of donor organs, and possible immunological cross-reactions.
Thus, for example, spinal cord transplantation is neither clinically nor biologically feasible and consequently there is no treatment available for SCI patients, while in the United States alone there are 250,000 chronically paralyzed patients with an increase of 10,000 new SCI patients each year.
On the other hand, implantation, transplantation or injection of cells into the body to replace or restore missing cells or part of tissue organs cannot properly achieve formation of new tissues because of the lack of a supporting extracellular matrix as a necessary tissue framework for tissue expansion and organization into an integrated structure in contact with the host organ. In addition, the cells need to be placed in a physiologically-equivalent environment that facilitate diffusion of nutrients, oxygen, humoral and cellular components in order to maintain high cell viability and growth potential after implantation.
Porous hydrogels of the present invention are deformable porous polymer matrices saturated with interstitial fluid or water, and thus provide the necessary tissue framework and hydrated space through which the cells can proliferate and assemble into supracellular tissue architectures in a correct histological structure to obtain a functional neotissue.
Different experimental strategies of intraspinal transplantation have been disclosed in the literature as attempts to restore damages in the spinal cord (animal models), using various implant materials which can be grouped into two broad categories of implants: (1) biological tissues and (2) prosthetic materials.
In category (1) is included the use of donor tissue grafts, either syngenic autograft or homograft, allograft or xenograft, to bridge lesions of spinal cord such as fetal neural tissue, either as (a) a solid graft (e.g. Bregman, Dev. Brain Res., 34, 265, 1987; Houle and Reier, J. Comp. Neurol., 269, 535, 1988) or as (b) suspension grafts including mixed neural tissue cells (e.g. Goldberg and Bernstein, J. Neuroscience Res., 19, 34, 1988; Hoovler and Wrathall, Acta Neuropathol., 81, 303, 1991); Schwann cells recombined with cultured sensory neurons (Kuhlengel et al., J. Comp. Neurol., 293, 74, 1990); immature astrocytes (e.g., Bernstein and Goldberg, Res. Neurol. Neurosci., 2, 261, 1991); precursors of neural tissue cells (Monteros et al., Dev. Neurosci., 14, 98, 1992) and immortalized established cell lines (Zompa et al., Int. J. Dev. Neurosci., 11, 535, 1993); peripheral nerve segment including cultured non-neuronal cells (Wrathall et al, Acta Neuropathol., 57, 59, 1982) or with embryonic neural tissue (Horvat et al., Res. Neurol. Neurosci., 2, 289, 1991). For category (2) prosthetic materials which have been disclosed include pure collagen matrices (de la Torre and Goldsmith, Brain Res., Bull., 35, 418, 1994; Marchand and Woerly, Neurosci. 36, 45, 1990; Gelderg, Brain Res. 511, 80, 1990), containing neuroactive agents (Goldsmith and de la Torre, Brain Res., 589, 217, 1992) or including cultured neural grafts (Bernstein and Goldberg, Brain Res. 377, 403, 1986); treated nitrocellulose implants (Schreyer and Jones, Dev. Brain Res., 35, 291, 1987; Houle and Johnson, Neurosci. Lett. 103, 17, 1989); collagen implants (Paino et al., J. Neurocytol., 23, 433, 1991) and polymer guidance channels of poly(acrylonitrile-vinyl chloride) (Xu et al., J. Comp. Neurol., 351, 145, 1995) enclosing Schwann cells.
These approaches focus very sharply on the promotion of axonal regeneration using various tissue substrates as sources of new axons or using complex prosthetic substrates to support and guide growing axons, and do not address the clinically relevant issue of spinal cord or brain tissue repair by regeneration of the bulk of the host tissue and remodeling of wound healing, for example, after removing necrotic or scar tissue following injury.
Polymer hydrogels have been disclosed as implants in the nervous system (Woerly et al., Biomaterials, 11, 97, 1990; Woerly et al., Biomaterials, 12, 197, 1991; Woerly et al., J. Neural Transpl. Plast. 3, 21, 1992; Woerly et al., Cell Transpl., 2, 229, 1993; Woerly et al., J. Neural Transpl. Plast., 5, 245, 1995). These hydrogels were prepared by free radical polymerization in water, using ammonium persulfate and sodium metabisulfite or persulfate and ascorbic acid as redox initiators with either hydroxyethyl methacrylate (pHEMA), glycidyl methacrylate pGMA) or N-hydroxypropyl methacrylamide (pHPMA) or a composition including the above monomers with a cross-linking agent which is either ethylene glycol and tetraethylene glycol dimethacrylate or methylene-bis-acrylamide. These gels are typically homogeneous and optically transparent with a bimodal porosity including open (accessible pore volume) and closed pores as shown by mercury porosimetry data and scanning electron microscopy; typically the porous structure for these gels is formed of parallel cylindrical capillaries of circular cross-section as shown in FIG. 1 with an average pore diameter of 7 to 13 .mu.m. The fractional porosity is in the range of 50% to 85% for pHEMA hydrogels, 60% to 65% for pGMA hydrogels and 70% to 94% for pHPMA hydrogels. At least 50% of the porous volume of the hydrogel is occupied by pore from 1.2 to 4 .mu.m for pHEMA, 6 to 13 .mu.m for pGMA and 10 to 14 .mu.m for pHPMA. It was found that their biological activity was dependent upon the introduction or copolymerization of collagen into the cross-linked network. Applicant experimented implantation in the brain which showed that some degree of tissue repair can be achieved according to the degree of tissue ingrowth into the homogeneous gel matrices. This reaction is variable according to the monomer composition and added functional groups. However, homogeneous hydrogels frequently induce the formation of fibrous capsule that tend to isolate the implant from the host. This is due to the mechanical properties of these gels that do not match sufficiently those of the living neural tissue as well as to the small volume fraction of macropores. In the spinal cord, these homogeneous hydrogels do not integrate into the host and become rapidly encapsulated by a connective tissue and glial scar without penetration of axons or tissue components, as shown in FIG. 2. In addition, there is a physical consideration that limits the surface area that can be generated, an important parameter for successful tissue interaction generated by the cylindrical pores in such homogeneous gels. For a fixed volume of gel, the surface area reaches a limit which is the maximum radius of a single pore occupying the total volume of the gel. On the other hand, increasing the surface area by decreasing the size of pores will lead to a decrease of the total void volume which is incompatible with tissue ingrowth and biomass accumulation.
Harvey et al., in Brain Res., 671, 119, 1995 discloses a polymer sponge of poly(2-hydroxyethyl methacrylate) that is used as brain implant for tissue regeneration and axon growth. This product is best used with the addition of collagen to the polymer network as tissue bioadhesive and after inclusion of Schwann cells.
U.S. Pat. No. 4,902,295 describes a process for preparing an artificial tissue from pancreas tissue cells. The process involves the polymerization of matrix precursors, gel precursors and promoters with viable cells in aqueous phase. All polymer precursors as well as promotors are biological compounds susceptible of rapid biodegration into the body and do not have long term stability after implantation.
Bellamkonda, R.; Ranieri, J. P.; Bouche, N.; Aebischer, P. ("Hydrogel-Based Three-dimensional Matrix for Neural Cells", J. Biomed. Mat. Res. 1995, 29, 663-671) describe a technique to immobilize neural tissue cells into agarose and extracellular-equivalent (Matrigel.RTM.) gels. These materials are biologic and are biodegradable.
Krewson, C. E.; Chung, S. W.; Dai, W.; Saltzman, W. M. ("Cell Aggregation and Neurite Growth in Gels of Extracellular Matrix Molecules". Biotechnol. Bioeng. 1994, 43, 555-562) describe a technique where PC12 cells are suspended in gels of collagen alone or combined with fibronectin or laminin, and in gels of agarose and collagen. These gels are biodegradable.
Cascone, M. G.; Laus, M.; Ricci, D.; Sbarbati del Guerra, R. ("Evaluation of Poly(vinyl alcohol) Hydrogels as a Component of Hybrid Artificial Tissues", J. Mat. Sci. Mat. Med. 1995, 6, 71-75) describe a technology using poly (vinyl alcohol) hydrogels, physically cross-linked, into which fibroblastic cells are introduced by a one freeze-thawing cycle.
Wald, H. L.; Sarakinos, G.; Lyman, M. D.; Mikos, A. G.; Vacanti, J. P.; Langer, R. ("Cell Seeding in Porous Transplantation", Biomat. 1993, 14, 270-278) describe a process for enclosing hepatocyte cells into degradable polymer foams of poly(L-lactic acid) by a microinjection technique. This technique does not provide a non-degradable matrix and does not allow uniform cell distribution throughout the polymer matrix.
Mikos, A. G.; Bao, Y.; Cima, L. G.; Ingber, D. E.; Vacanti, J. P.; Langer, R. ("Preparation of Poly(glycolic acid) Bonded Fiber Structures for Cell Attachment and Transplantation", J. Biomed. Mat. Res. 1993, 27 183-189) describe a process to build networks of poly(glycolic acid) with bonded fibers to culture hepatocytes. This polymer is biodegradable and the process to introduce cells into the matrix is different from entrapment.
Puerlacher, W. C.; Mooney, D.; Langer, R.; Upton, J.; Vacanti, J. P.; Vacanti, C. A. ("Design of Nasoseptal Cartilage Replacements Synthetized from Biodegradable Polymers and Chondrocytes", Biomat. 1994, 15, 774-778) and Freed, L. E.; Marquis, J. C.; Nohria, A. Emmanual; Mikos, A. G.; Langer, R. ("Neocartilage Formation In Vitro and In Vivo Using Cells Cultured on Synthetic Biodegradable Polymers", J. Biomed. Mat. Res. 1993 27, 11-23). These references describe a process to introduce chondrocyte cells into polyglycolic (PGA) or polylactic acid (PLLA) or PGA-PLLA matrices by capillary action. This process yields biodegradable polymer materials while the cells are not uniformly distributed into the polymer and does not allow to control cell density.
Cao, Y.; Vacanti, J. P.; Ma, X.; Paige; K. T.; Upton, J.; Chowanski, Z.; Schloo, B.; Langer, R.; Vacanti, C. A. ("Generation of Neo-Tendon Using Synthetic Polymers Seeded with Tenocytes", Transpl. Proc 1994, 26, 3390-3391) describe a process to seed tenocyte cells into embossed nonwoven mesh of polyglycolic acid.
Mooney, D. J.; Park, S.; Kaufman, P. M.; Sano, K.; McNamara, K.; Vacanti, J. P.; Langer, R. ("Biodegradable Sponge for Hepatocyte Transplantation", J. Biomed. Mat. Res 1995, 29, 959-965) and Takeda, T.; Kim, T. H.; Lee, S. K.; Langer, R.; Vacanti, J. O. ("Hepatocyle Transplantation in Biodegradable Polymer Scaffolds Using the Baltimatian Dog model of Hyperuricosuria", Transpl. Proc. 1995, 27, 635-636) describe a process to absorb hepatocyte cells in sheets of polyglycolic acid polymer felts or into polymer sponges fabricated from polylactic acid and polyvinyl alcohol and from polylactic acid glycolic acid by adsorption and capillary action. This process yields biodegradable polymer materials while the cells are not uniformly distributed into the polymer and does not allow to control cell density.
Woerly, S.; Plant, G. W.; Harvey, A. R. ("Cultured Rat Neuronal and Glial Cells Entrapped within Hydrogel Polymer Matrices: A Potential Tool for Neural Tissue Replacement", Neurosci. Lett. 1996, 205, 197-201) disclose a procedure to entrap neural tissue cells into homogeneous transparent polymer gels of polyN-(2-hydroxypropyl)-methacrylamide! which can contain collagen as attachment substrate. This procedure involves the addition of a cell suspension to the polymer mixture and the polymerization of the cell-polymer mixture at room temperature or in an incubator maintained at 37.degree. C. The resulting gel is optically transparent and cells are randomly dispersed within the cross-linked gel. Immunocytochemical studies indicated that cell viability after 6 days in vitro varied between 0 and 6%.